[Build][CMake] Use CMake internal instead of custom command for moc
[Build][CMake] Use CMake internal instead of custom command for moc

CMake has an internal function (`qt5_generate_moc`) that can be used to
forcibly generate a `.moc` file. Use that instead of the generic
`execute_process` function.

This fixes an issue on systems (fe, Arch Linux) where Qt's moc binary
are placed in a different location than debian based distros.

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